Error on pointless class attribute

hypothesis-python/RELEASE.rst

@@ -1,4 +1,15 @@
-RELEASE_TYPE: patch
+RELEASE_TYPE: minor
 
-This patch updates our vendored `list of top-level domains <https://www.iana.org/domains/root/db>`__,
-which is used by the provisional :func:`~hypothesis.provisional.domains` strategy.
+This release makes it an error to assign ``settings = settings(...)``
+as a class attribute on a :class:`~hypothesis.stateful.RuleBasedStateMachine`.
+This has never had any effect, and it should be used as a decorator instead:
+
+.. code-block: python
+
+    class BadMachine(RuleBasedStateMachine):
+        """This doesn't do anything, and is now an error!"""
+        settings = settings(derandomize=True)
+
+    @settings(derandomize=True)
+    class GoodMachine(RuleBasedStateMachine):
+        """This is the right way to do it :-)"""

hypothesis-python/src/hypothesis/stateful.py

@@ -220,9 +220,10 @@ def run_state_machine(factory, data):
 class StateMachineMeta(type):
     def __setattr__(cls, name, value):
         if name == "settings" and isinstance(value, Settings):
+            descr = f"settings({value.show_changed()})"
             raise AttributeError(
-                f"Assigning {cls.__name__}.settings = {value} does nothing. Assign "
-                f"to {cls.__name__}.TestCase.settings, or use @{value} as a decorator "
+                f"Assigning {cls.__name__}.settings = {descr} does nothing. Assign "
+                f"to {cls.__name__}.TestCase.settings, or use @{descr} as a decorator "
                 f"on the {cls.__name__} class."
             )
         return super().__setattr__(name, value)
@@ -255,6 +256,15 @@ def __init__(self) -> None:
         self._initialize_rules_to_run = copy(self.initialize_rules())
         self._rules_strategy = RuleStrategy(self)
 
+        if isinstance(s := vars(type(self)).get("settings"), Settings):
+            tname = type(self).__name__
+            descr = f"settings({s.show_changed()})"
+            raise InvalidDefinition(
+                f"Assigning settings = {descr} as a class attribute does nothing. "
+                f"Assign to {tname}.TestCase.settings, or use @{descr} as a decorator "
+                f"on the {tname} class."
+            )
+
     def _pretty_print(self, value):
         if isinstance(value, VarReference):
             return value.name

hypothesis-python/tests/cover/test_stateful.py

@@ -1215,3 +1215,19 @@ def test_min_steps_argument():
 
     # (oh, and it's OK if you ask for more than we're actually going to take)
     run_state_machine_as_test(MinStepsMachine, _min_steps=20)
+
+
+class ErrorsOnClassAttributeSettings(RuleBasedStateMachine):
+    settings = Settings(derandomize=True)
+
+    @rule()
+    def step(self):
+        pass
+
+
+def test_fails_on_settings_class_attribute():
+    with pytest.raises(
+        InvalidDefinition,
+        match="Assigning .+ as a class attribute does nothing",
+    ):
+        run_state_machine_as_test(ErrorsOnClassAttributeSettings)